Liquid crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterised in that it comprises one or more compounds of the formula I 
                         
and one or more compounds of the formula IA
 
                         
in which R 1 , R 2  ring A, ring B, L 1-4 , Z 1 , Z 2 , X 1 , X 2  and a are as defined in claim  1,    
where the proportion of the compounds of the formula I in the medium is at least 18% by weight.

The present invention relates to a liquid-crystalline medium, to the usethereof for electro-optical purposes, and to displays containing thismedium.

Liquid crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (super-birefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and lower vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

-   1. MOS (metal oxide semiconductor) or other diodes on a silicon    wafer as substrate.-   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out worldwide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are back-lit.

The term MLC displays here covers any matrix display with integratednon-linear elements, i.e., besides the active matrix, also displays withpassive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TVs) or for high-information displays for computerapplications (laptops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due toinsufficiently high specific resistance of the liquid-crystal mixtures[TOGASHI, S., SEKO-GUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p.141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Designof Thin Film Transistors for Matrix Addressing of Television LiquidCrystal Displays, p. 145 if, Paris]. With decreasing resistance, thecontrast of an MLC display deteriorates, and the problem of after-imageelimination may occur. Since the specific resistance of theliquid-crystal mixture generally drops over the life of an MLC displayowing to interaction with the interior surfaces of the display, a high(initial) resistance is very important in order to obtain acceptableservice lives. In particular in the case of low-volt mixtures, it washitherto impossible to achieve very high specific resistance values. Itis furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures fromthe prior art are also particularly disadvantageous. It is demanded thatno crystallisation and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not meettoday's requirements.

There thus continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times even at low temperatures and a low thresholdvoltage which do not have these disadvantages, or only do so to areduced extent.

In addition to liquid-crystal displays which use back-lighting, i.e. areoperated transmissively and if desired transflectively, reflectiveliquid-crystal displays are also particularly interesting. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than back-litliquid-crystal displays having a corresponding size and resolution.Since the TN effect is characterised by very good contrast, reflectivedisplays of this type can even be read well in bright ambientconditions. This is already known of simple reflective TN displays, asused, for example, in watches and pocket calculators. However, theprinciple can also be applied to high-quality, higher-resolution activematrix-addressed displays, such as, for example, TFT displays. Here, asalready in the transmissive TFT-TN displays which are generallyconventional, the use of liquid crystals of low birefringence (Δn) isnecessary in order to achieve low optical retardation (d•Δn). This lowoptical retardation results in usually acceptable low viewing-angledependence of the contrast (cf. DE 30 22 818). In reflective displays,the use of liquid crystals of low birefringence is even more importantthan in transmissive displays since the effective layer thicknessthrough which the light passes is approximately twice as large inreflective displays as in transmissive displays having the same layerthickness.

In TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages-in the-cells:

-   -   extended nematic phase range (in particular down to low        temperatures)    -   storage stability, even at extremely low temperatures    -   the ability to switch at extremely low temperatures (outdoor        use, automobiles, avionics)    -   increased resistance to UV radiation (longer service life)    -   low optical birefringence (An) for reflective displays.

The media available from the prior art do not allow these advantages tobe achieved while at the same time retaining the other parameters. Themixtures disclosed in EP 1 046 693 A1 and EP 1 046 694 A1 aredistinguished by high viscosities.

In the case of supertwisted (STN) cells, media are desired which enablegreater multiplexability and/or a lower threshold voltage and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther widening of the available parameter latitude (clearing point,smectic-nematic transition or melting point, viscosity, dielectricparameters, elastic parameters) is urgently desired.

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not have the above-mentioneddisadvantages or only do so to a reduced extent, and preferablysimultaneously have very low threshold voltages, low viscosities andhigh values for the voltage holding ratio (VHR).

It has now been found that this object can be achieved if mediaaccording to the invention are used in displays.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds of positive dielectric anisotropy,characterised in that it comprises one or more compounds of the formulaI

and one or more compounds of the formula IA

where the proportion of compounds of the formula I is at least 18% byweight, and in which the individual radicals have the followingmeanings:

-   R¹ is an alkenyl radical having from 2 to 8 carbon atoms,-   R² is H, an alkyl radical having from 1 to 15 carbon atoms which is    halogenated, substituted by CN or CF₃ or unsubstituted, where, in    addition, one or more CH₂ groups in these radicals may each,    independently of one another, be replaced by —C≡C—, —CO—, —CH═CH—,    —O—,

-    or

-    in such a way that O atoms are not linked directly to one another,-   X¹ is an alkyl radical, alkenyl radical, alkoxy radical or    alkenyloxy radical, each having up to 6 carbon atoms, in the case    where a=1 also F, Cl, CN, SF₅, SCN, NCS or OCN,-   X² is F, Cl, CN, SF₅, SCN, NCS, OCN, a halogenated alkyl radical,    halogenated alkenyl radical, halogenated alkoxy radical or    halogenated alkenyloxy radical, each having up to 6 carbon atoms,-   Z¹ and Z² are each, independently of one another, —CF₂O—, —OCF₂— or    a single bond, where Z¹≠Z²,

-   and

-    are each, independently of one another,

-   a is 0 or 1, and-   L¹⁻⁴ are each, independently of one another, H or F.

The compounds of the formulae I and IA have a broad range ofapplications. Depending on the choice of substituents, these compoundscan serve as base materials of which liquid-crystalline media arepredominantly composed; however, it is also possible to add compounds ofthe formulae I and IA to liquid-crystalline base materials from otherclasses of compound in order, for example, to modify the dielectricand/or optical anisotropy of a dielectric of this type and/or in orderto optimise its threshold voltage and/or its viscosity. The mixingconcept according to the invention results in mixtures which aredistinguished over the prior art by their very good reliability andV_(th)/γ₁ ratio, in particular in 2.5 V and 3.3 V mixtures. The mixturesaccording to the invention are particularly suitable for notebook PC,PDA and other mobile applications.

In the pure state, the compounds of the formulae I and IA are colourlessand form liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

R¹ in the formula I is preferably vinyl, 1E-alkenyl or 3-alkenyl.

If R² in the formula IA is an alkyl radical and/or an alkoxy radical,this may be straight-chain or branched. It is preferably straight-chain,has 1, 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly is preferablymethyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, methoxy, ethoxy,propoxy, butoxy, pentoxy, hexyloxy or heptyloxy, furthermore octyl,nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy, tridecyloxy ortetradecyloxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6-, or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R² is an alkyl radical in which one CH₂ group has been replaced by—CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has from 2 to 10 carbon atoms. Accordingly, it is inparticular vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-,-3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl,non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-,-4-, -5-, -6-, -7-, -8- or -9-enyl.

If R² is an alkyl radical in which one CH₂ group has been replaced by—O— and one has been replaced by —CO—, these are preferably adjacent.These thus contain an acyloxy group —CO—O— or an oxycarbonyl group—O—CO—. These are preferably straight-chain and have from 2 to 6 carbonatoms. Accordingly, they are in particular acetoxy, propionyloxy,butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl,propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl,2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 2-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R² is an alkyl radical in which one CH₂ group has been replaced byunsubstituted or substituted —CH═CH— and an adjacent CH₂ group has beenreplaced by CO or CO—O or O—CO, this may be straight-chain or branched.It is preferably straight-chain and has from 4 to 12 carbon atoms.Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl,3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl,6-acryloyloxyhexyl, 7-acryloyloxyheptyl, 8-acryloyloxyoctyl,9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl,2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-methacryloyloxybutyl,5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or9-methacryloyloxynonyl.

If R² is an alkyl or alkenyl radical which is monosubstituted by CN orCF₃, this radical is preferably straight-chain. The substitution by CNor CF₃ is in any desired position.

If R² is an alkyl or alkenyl radical which is at least monosubstitutedby halogen, this radical is preferably straight-chain, and halogen ispreferably F or Cl. In the case of polysubstitution, halogen ispreferably F. The resultant radicals also include perfluorinatedradicals. In the case of mono-substitution, the fluorine or chlorinesubstituent may be in any desired position, but is preferably in theco-position.

Compounds containing branched wing groups R² may occasionally be ofimportance owing to better solubility in the conventionalliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R² are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

If R² is an alkyl radical in which two or more CH₂ groups have beenreplaced by —O— and/or —CO—O—, this may be straight-chain or branched.It is preferably branched and has from 3 to 12 carbon atoms.Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.

The compounds of the formulae I and IA are prepared by methods known perse, as described in the literature (for example in the standard works,such as Houben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants which are known per se, but arenot mentioned here in greater detail. The compounds of the formula IAare disclosed, for example, in EP 1 046 693 A1 and EP 1 046 694 A1. Thecompounds of the formula I are described, for example, in EP 0 122 389B1.

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates, which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance which is located in the cell) which contain media ofthis type, and to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant widening of the available parameter latitude. The achievablecombinations of clearing point, viscosity at low temperature, thermaland UV stability and dielectric anisotropy are far superior to previousmaterials from the prior art.

Compared with the mixtures disclosed in EP 1 046 694 A1, the mixturesaccording to the invention have a higher clearing point, low γ₁ values,lower values for the flow viscosity and very high values for the VHR at100° C. The mixtures according to the invention are preferably suitableas TN-TFT mixtures for notebook PC applications with 3.3 and 2.5 Vdrivers.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −30° C., particularly preferably down to −40°C., enable clearing points above 70° C., preferably above 75° C.,particularly preferably ≧80° C., simultaneously dielectric anisotropyvalues Δε of ≧6, preferably ≧8, and a high value for the specificresistance to be achieved, enabling excellent STN and MLC displays to beobtained. In particular, the mixtures are characterised by low operatingvoltages. The TN thresholds are below 1.5 V, preferably below 1.4 V,particularly preferably <1.3 V.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 110° C.) to be achieved athigher threshold voltages or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having greater Δε and thus lowthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German Patent 30 22 818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistance values to be achieved using the mixturesaccording to the invention at the first minimum than in the case ofmixtures comprising cyano compounds. Through a suitable choice of theindividual components and their proportions by weight, the personskilled in the art is able to set the birefringence necessary for apre-specified layer thickness of the MLC display using simple routinemethods.

The flow viscosity ν₂₀ at 20° C. is preferably <20 mm²·s⁻¹, particularlypreferably <19 mm²•s⁻¹. The rotational viscosity γ₁ at 20° C. of themixtures according to the invention is preferably <140 mPa·s,particularly preferably <120 mPa·s. The nematic phase range ispreferably at least 100°, in particular at least 110°. This rangepreferably extends at least from −40° to +80°.

A short response time is desired in liquid-crystal displays. Thisapplies in particular to displays which are capable of videoreproduction. For displays of this type, response times (sum:t_(on)+t_(off)) of at most 16 ms are required. The upper limit of theresponse time is determined by the image refresh frequency.

Measurements of the voltage holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formulae I and IA exhibit asignificantly smaller decrease in the HR with increasing temperaturethan analogous mixtures comprising cyanophenylcyclohexanes of theformula

or esters of the formula

instead of the compounds of the formula IA.

The mixtures according to the invention preferably comprise little(≦20%, in particular ≦10%) or no nitrites. The holding ratio of themixtures according to the invention at 20° C. is at least 98%,preferably >99%. The UV stability of the mixtures according to theinvention is also considerably better, i.e. they exhibit a significantlysmaller decrease in the HR on exposure to UV.

The formula I preferably encompasses compounds of the formulae I-1 toI-5

Particularly preferred compounds of the formula I are compounds of theformulae

in which

-   n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12, preferably 1, 2, 3 or    5.

Alkyl is straight-chain alkyl having 1-15 carbon atoms, preferably CH₃,C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁, or n-C₆H₁₃.

Alkenyl is preferably CH₂═CH, CH₃CH═CH, CH₂═CH₂CH₂ or CH₃—CH═CHCH₂CH₂.

The rings A and B are preferably

Preference is given to media according to the invention which compriseat least one compound of the formulae I-1 and/or I-2.

Particularly preferred compounds of the formula IA are compounds of theformulae IA-1 to IA-30

in which R² is as defined above.

Of these preferred compounds, particular preference is given to those ofthe formulae IA-2, IA-3, IA-5, IA-6 and IA-14, IA-15 and IA-18, inparticular those of the formulae IA-3 and IA-15.

Preference is furthermore given to compounds of the formula IA whichcontain at least one heterocyclic ring, in particular compounds of theformulae

R² in the compounds of the formulae IA and IA-1 to IA-54 is preferablyH, straight-chain alkyl having from 1 to 7 carbon atoms, in particularCH₃, C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃, n-C₇H₁₅, furthermore 1E- or3-alkenyl, in particular CH₂═CH, CH₃CH═CH, CH₂═CHCH₂CH₂ orCH₃CH═CH—CH₂CH₂.

Preferred embodiments are indicated below:

-   -   The medium comprises one, two or more compounds selected from        the group consisting of the formulae IA-1 to IA-54;    -   The medium preferably comprises in each case one or more,        preferably two or three, compounds (homologues) of the formulae        I-1 and IA-15;    -   The medium preferably comprises in each case one or more,        preferably two or three, compounds (homologues) of the formulae        I-1 and IA-3;    -   The medium additionally comprises one or more compounds selected        from the group consisting of the general formulae II to VI

in which the individual radicals have the following meanings:

-   R⁰ is H, n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or    alkenyl, each having up to 9 carbon atoms,-   X⁰ is F, Cl, halogenated alkyl, alkenyl, alkenyloxy or alkoxy having    up to 6 carbon atoms,-   Z⁰ is —C₂F₄—, —CF═CF—, —CH═CF—, —CF═CH—, —CH═CH—, —O(CH₂)₃—,    —(CH₂)₃O—, —C₂H₄—, —(CH₂)₄—, −CF₂O—, —OCF₂—, —OCH₂— or —CH₂O—,-   Y¹⁻⁴ are each, independently of one another, H or F,-   r is 0 or 1,    where the compound of the formula II is not identical with the    compound of the formula I.

The compound of the formula IV is preferably

-   -   The medium additionally comprises one or more compounds selected        from the group consisting of the general formulae VII to XIII

in which R⁰, X⁰, Y¹ and Y² are each, independently of one another, asdefined in claim 4. Y³ and Y⁴ are each, independently of one another, Hor F. X⁰ is preferably F, Cl, CF₃, OCF₃ or OCHF₂. R⁰ is preferablyalkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 6 carbonatoms.

-   -   The medium additionally comprises one or more ester compounds of        the formulae Ea to Ef

in which R⁰ is as defined in claim 4;

-   -   The proportion of the compounds of the formulae Ea to Ef is        preferably 10-30% by weight, in particular 15-25% by weight;    -   The proportion of compounds of the formulae IA and I to VI        together in the mixture as a whole is at least 50% by weight;    -   The proportion of compounds of the formula I in the mixture as a        whole is ≧18% by weight, preferably ≧20% by weight, in        particular ≧22% by weight, very particularly preferably ≧24% by        weight;    -   The proportion of compounds of the formula IA in the mixture as        a whole is from 5 to 40% by weight, particularly preferably from        10 to 30% by weight;    -   The proportion of compounds of the formulae II to VI in the        mixture as a whole is from 30 to 8.0% by weight;

-   -   is preferably

-   -   The medium comprises compounds of the formula II, III, IV, V or        VI;    -   R⁰ is straight-chain alkyl or alkenyl having from 1 or 2 to 7        carbon atoms respectively;    -   The medium essentially consists of compounds of the formulae IA        and I to VI;    -   The medium comprises further compounds, preferably selected from        the following group consisting of the general formulae XIV to        XVII

in which R⁰ and X⁰ are as defined above, and the 1,4-phenylene rings maybe substituted by CN, chlorine or fluorine. The 1,4-phenylene rings arepreferably monosubstituted or polysubstituted by fluorine atoms.

-   -   The medium additionally comprises one or more compounds of the        formulae XVIII and/or XIX

in which R⁰, X⁰, Y¹ and Y² are as defined above.

The proportion of the compounds of the formula XVIII in the mixtureaccording to the invention is 2-30% by weight, while the compounds ofthe formula XIX may be present in the mixture in amounts of 0.5-30% byweight. The compounds of the formula XIX also simultaneously have apositive influence on the UV stability of the mixtures.

-   -   The medium additionally comprises one, two, three or more,        preferably two or three, compounds of the formulae

in which “alkyl” and “alkyl*” are each, independently of one another, astraight-chain or branched alkyl radical having 1-9 carbon atoms.

The proportion of the compounds of the formulae O1 and/or O2 in themixtures according to the invention is preferably 5-10% by weight.

-   -   The medium preferably comprises 5-35% by weight of compound IVa.    -   The medium preferably comprises one, two or three compounds of        the formula IVa in which X⁰ is F or OCF₃.    -   The medium preferably comprises one or more compounds of the        formulae IIa to IIg

in which R⁰ is as defined above. In the compounds of the formulaeIIa-IIg, R⁰ is preferably H, methyl, ethyl, n-propyl, n-butyl orn-pentyl, furthermore n-hexyl or n-heptyl.

-   -   The (I+IA): (II+IlIl+IV+V+VI) weight ratio is preferably from        1:10 to 10:1.    -   The medium essentially consists of compounds selected from the        group consisting of the general formulae IA and I to XIII.    -   The proportion of the compounds of the formulae IVb and/or IVc        in which X⁰ is fluorine and R⁰ is C₂H₅, n-C₃H₇, n-C₄H₉ or        n-C₅Ha₁₁ in the mixture as a whole is from 2 to 20% by weight,        in particular from 2 to 15% by weight;    -   The medium preferably comprises one, two or three, furthermore        four, homologues of the compounds selected from the group        consisting of H1 to H19 (n=1-12)

-   -   The medium preferably comprises compound IIb in which R⁰ is        methyl;    -   The medium comprises low Δn compounds, preferably selected from        the following group consisting of the formulae RI to RVII

in which

-   R* is n-alkyl, alkoxy, oxaalkyl, fluoroalkyl-or alkenyloxy, each    having up to 9 carbon atoms, and alkyl and-   alkyl* are each, independently of one another, a straight-chain or    branched alkyl radical having 1-9 carbon atoms.    -   Preferred mixtures for TN monitor applications comprise        compounds of the formula

in which

-   R* is as defined above and is preferably alkenyl, in particular    CH₃CH═CH.

The benzene ring may additionally be substituted by one or two fluorineatoms.

-   -   The medium additionally comprises one, two or more compounds        containing fused rings of the formulae AN1 to AN11

in which R⁰ is as defined above.

The term “alkyl” or “alkyl*” covers straight-chain and branched alkylgroups having 1-7 carbon atoms, in particular the straight-chain groupsmethyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having2-5 carbon atoms are generally preferred.

The term “alkenyl” covers straight-chain and branched alkenyl groupshaving 2-7 carbon atoms, in particular the straight-chain groups.Preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl. Examples ofparticularly preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

-   -   The medium additionally comprises one or more pyran compounds of        the formulae P-1 to P-11

in which R⁰ is as defined above.

-   -   The medium preferably comprises one or more dioxane compounds of        the formulae D-1 and/or D-2

in which

-   -   R⁰ is as defined above.    -   Preferred mixtures comprise 2-30% of dioxanes.

The term “fluoroalkyl” preferably covers straight-chain groups having aterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. Preferably, n=1 and m is from1 to 6.

It has been found that even a relatively small proportion of compoundsof the formulae I and IA mixed with conventional liquid-crystalmaterials, but in particular with one or more compounds of the formulaeII, III, IV, V and/or VI, results in a significant lowering of thethreshold voltage and in high-values for-the VHR (100° C.), with broadnematic phases with low smectic-nematic transition temperatures beingobserved at the same time, improving the shelf life. Preference isgiven, in particular, to mixtures which, besides one or more compoundsof the formulae I and IA, comprise one or more compounds of the formulaIV, in particular compounds of the formula IVa in which X⁰ is F or OCF₃.The compounds of the formulae IA and I to VI are colourless, stable andreadily miscible with one another and with other liquid-crystalmaterials.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio between the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxyradicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and smaller values of k₃₃/k₁₁ comparedwith alkyl and alkoxy radicals.

A —CH₂CH₂— group generally results in higher values of k₃₃/k₁₁ comparedwith a single covalent bond. Higher values of k₃₃/k₁₁ facilitate, forexample, flatter transmission characteristic lines in TN cells with a90° twist (in order to achieve grey shades) and steeper transmissioncharacteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

The optimum mixing ratio of the compounds of the formulae I, IA andII+III+IV+V+VI depends substantially on the desired properties, on thechoice of the components of the formulae I, IA, II, III, IV, V and/orVI, and on the choice of any further components that may be present.Suitable mixing ratios within the range given above can easily bedetermined from case to case.

The total amount of compounds of the formulae IA and I to XIII in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimisation of various properties. However, the observed effect on theaddressing times and the threshold voltage is generally greater, thehigher the total concentration of compounds of the formulae IA and I toXIII.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VI (preferably II,III and/or IV, in particular IVa) in which X⁰ is F, OCF₃, OCHF₂,OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergistic effect with thecompounds of the formulae I and IA results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaeI and IA and of the formula IVa are distinguished by their low thresholdvoltages.

The individual compounds of the formulae IA and I to XVIII and theirsub-formulae which can be used in the media according to the inventionare either known or can be prepared analogously to the known compounds.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the usual design for displays of this type. The termusual design is broadly drawn here and also covers all derivatives andmodifications of the MLC display, in particular including matrix displayelements based on poly-Si TFTs or MIM.

A significant difference between the displays according to the inventionand the hitherto conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se. In general, thedesired amount of the components used in lesser amount is dissolved inthe components making up the principal constituent, advantageously atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and to remove the solvent again, for example by distillation,after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature, such as, forexample, stabilisers, UV filters and antioxidants. For example, 0-15% ofpleochroic dyes or chiral dopants can be added.

C denotes a crystalline phase, S a smectic phase, Sc a smectic C phase,N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2.0 times the value of V₁₀. Δn denotes the optical anisotropy. Δεdenotes the dielectric anisotropy (Δε =ε_(∥), ε_(⊥), where ε_(∥) denotesthe dielectric constant parallel to the longitudinal molecular axes andε_(⊥) denotes the dielectric constant perpendicular thereto). Theelectro-optical data are measured in a TN cell at the 1st minimum (i.e.at a d•Δn value of 0.5 μm) at 20° C., unless expressly stated otherwise.The optical data are measured at 20° C., unless expressly statedotherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and CmH_(2m+1) arestraight-chain alkyl radicals having n and m carbon atoms respectively;n and m are integers and are preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11 or 12. The coding in Table B is self-evident. In Table A, onlythe acronym for the parent structure is indicated. In individual cases,the acronym for the parent structure is followed, separated by a dash,by a code for the substituents R^(1*), R^(2*), L^(1*), L^(2*) andL^(3*):

Code for R^(1*), R^(2*), L^(1*), L^(2*), L^(3*) R^(1*) R^(2*) L^(1*)L^(2*) L^(3*) nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H H nOm OC_(n)H_(2n+1)C_(m)H_(2m+1) H H H nO.m C_(n)H_(2n+1) OC_(m)H_(2m+1) H H H nC_(n)H_(2n+1) CN H H H nN.F C_(n)H_(2n+1) CN H H F nN.F.F C_(n)H_(2n+1)CN H F F nF C_(n)H_(2n+1) F H H H nOF OC_(n)H_(2n+1) F H H H nF.FC_(n)H_(2n+1) F H H F nmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H H nOCF₃C_(n)H_(2n+1) OCF₃ H H H nOCF₃.F C_(n)H_(2n+1) OCF₃ F H H n-VmC_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H H nV-Vm C_(n)H_(2n+1)—CH═CH——CH═CH—C_(m)H_(2m+1) H H H

Preferred mixture components are given in Tables A and B.

TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-n-F

CGG

CGU

CFU

TABLE B

BCH-n.Fm

CFU-n-F

CBC-nmF

ECCP-nm

CCZU-n-F

T-nFm

CGU-n-F

CDU-n-F

DCU-n-F

CGG-n-F

CPZG-n-OT

CC-nV-Vm

CCP-Vn-m

CCG-V-F

CCP-nV-m

CC-n-V

CCQU-n-F

CC-n-V1

CCGG-n-F

CQCU-n-F

Dec-U-n-F

CWUC-n-F

CWCG-n-F

CCOC-n-m

CPTU-n-F

GPTU-n-F

PQU-n-F

PUQU-n-F

PGU-n-F

CCP-nCl

CGZP-n-OT

CCCU-n-F

CCGU-n-F

CCPU-n-F

CECU-n-F

CCEU-n-F

CCQG-n-F

CUQG-n-F

CUQU-n-F

CGUQU-n-F

CPUQU-n-F

CGUQU-n-OT

AUUQU-n-F

AGUQU-n-F

ACQU-n-F

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formulae I and IA, comprise at least one,two, three or four compounds from Table B.

Table C:

Table C shows possible dopants which are generally added to the mixturesaccording to the invention.

TABLE C Table C shows possible dopants which are generally added to themixtures according to the invention.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 44

CM 47

R/S-1011

R/S-3011

R/S-4011

CN

R/S-2011

R/S-5011Table D:

Stabilisers which can be added, for example, to the mixtures accordingto the invention are mentioned below.

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention are mentioned below.

The following examples are intended to explain the invention withoutrestricting it. Above and below, percentages are per cent by weight. Alltemperatures are indicated in degrees Celsius. m.p. denotes meltingpoint, cl.p. denotes clearing point. Furthermore, C=crystalline state,N=nematic phase, S=smectic phase and I=isotropic phase. The data betweenthese symbols represent the transition temperatures. Δn denotes opticalanisotropy (589 nm, 20° C.), Δε denotes the dielectric anisotropy (1kHz, 20° C.). The flow viscosity ν₂₀ (mm²/sec) is determined at 20° C.The rotational viscosity γ₁ (mPa·s) is likewise determined at 20° C.

EXAMPLE M1

PUQU-2-F 3.00% Clearing point [° C.]: 70.1 PCH-7F 5.00% Δn [589 nm, 20°C.]: 0.0832 CCP-2F.F.F 12.00% Δε [1 kHz, 20° C.]: 8.7 CCP-1F.F.F 7.50% ν[mm²s⁻¹, 20° C.]: 17.3 CCP-30CF₃ 8.00% CCP-4F.F.F 10.00% CGUQU-2-F10.50% CCH-34 5.00% CPUQU-3-F 10.00% CC-5-V 15.00% CC-3-V1 8.00%CCP-20CF₃ 5.00% CCG-V-F 1.00%

EXAMPLE M2

CC-3-V 15.00% Clearing point [° C.]: 80.0 CCZU-2-F 4.00% Δn [589 nm, 20°C.]: 0.0899 CCZU-3-F 14.00% Δε [1 kHz, 20° C.]: 10.9 CGZP-2-OT 10.00% ν[mm²s⁻¹, 20° C.]: 19.1 CDU-2-F 7.00% CDU-3-F 8.00% CDU-5-F 7.00% PGU-2-F1.00% CCH-35 4.00% CGU-2-F 2.50% CC-3-V1 7.50% CPUQU-3-F 10.00%CPUQU-2-F 10.00%

EXAMPLE M3

CC-3-V 16.00% Clearing point [° C.]: 82.0 CC-3-V1 9.00% Δn [589 nm, 20°C.]: 0.0895 CCH-35 4.00% Δε [1 kHz, 20° C.]: 11.0 CC-5-V 2.50% ν[mm²s⁻¹, 20° C.]: 18.3 CCZU-2-F 4.00% CCZU-3-F 10.00% CGZP-2-OT 10.00%CGZP-3-OT 8.00% CDU-2-F 7.00% CDU-3-F 8.00% CDU-5-F 1.50% CGUQU-2-F10.00% CGUQU-3-F 10.00%

COMPARATIVE EXAMPLE (p. 24 of EP 1 046 694)

PUQU-2-F 9.00% Clearing point [° C.]: 72.1 PUQU-3-F 9.00% Δn [589 nm,20° C.]: 0.0841 PCH-7F.F.F 6.00% Δε [1 kHz, 20° C.]: 8.2 CECU-3-F 9.00%ν [mm²s⁻¹, 20° C.]: 21.6 CECU-4-F 9.00% CECU-5-F 9.00% CCP-3F.F.F 7.00%CCP-4F.F.F 5.00% CCEU-3-F 4.00% CCPU-3-F 3.00% PCH-302 3.00% CCH-3410.00% CCH-35 5.00% CCP-31 2.00% CCP-4CI 4.00% CCCU-2-F 2.00% CCCU-3-F2.00% CCCU-4-F 2.00%

EXAMPLE M4

CCP-20CF₃ 3.00% S → N [° C.]: <−40.0 CCP-30CF₃ 7.00% Clearing point [°C.]: +83.5 CCP-40CF₃ 3.00% Δn [589.3 nm, 20° C.]: 0.0805 CCZU-2-F 4.00%γ₁ [mPa · s, 20° C.]: 89 CCZU-3-F 13.00% d · Δn [20° C., μm]: 0.50CGZP-2-OT 8.00% Twist [° C.]: 90 CDU-2-F 9.00% V₁₀ [V]: 1.30 CDU-3-F6.00% CC-3-V1 12.00% CC-3-V 18.00% CCH-35 4.00% CGUQU-2-F 10.00%CGUQU-3-F 3.00%

EXAMPLE M5

CCP-20CF₃ 7.00% S → N [° C.]: <−40.0 CCP-30CF₃ 4.00% Clearing point [°C.]: +80.5 CCP-2F.F.F 3.00% Δn [589.3 nm, 20° C.]: 0.0800 CCZU-2-F 4.00%γ₁ [mPa · s, 20° C.]: 90 CCZU-3-F 13.00% d · Δn [20° C., μm]: 0.50CGZP-2-OT 6.00% Twist [° C.]: 90 CDU-2-F 7.00% V₁₀ [V]: 1.28 CDU-3-F7.00% CC-3-V1 11.00% CC-3-V 18.00% CCH-35 5.00% CGUQU-2-F 10.00%CGUQU-3-F 5.00%

EXAMPLE M6

CC-5-V 8.00% S → N [° C.]: <−30.0 CC-3-V1 11.00% Clearing point [° C.]:+85.0 CCH-35 5.00% Δn [589.3 nm, 20° C.]: 0.0960 CCP-1F.F.F 10.00% γ₁[mPa · s, 20° C.]: 128 CCP-2F.F.F 9.00% d · Δn [20° C., μm]: 0.50CCP-3F.F.F 2.00% Twist [° C.]: 90 CCP-30CF₃.F 5.00% V₁₀ [V]: 1.28CCP-20CF₃ 2.00% CCP-30CF₃ 8.00% CCP-40CF₃ 6.00% CGU-3-F 5.00% PGU-2-F6.00% CCP-V-1 3.00% CGUQU-2-OT 10.00% CGUQU-3-OT 10.00%

EXAMPLE M7

CCP-20CF₃ 3.00% S → N [° C.]: <−40.0 CCP-30CF₃ 7.00% Clearing point [°C.]: 77.5 CCP-40CF₃ 3.00% Δn [589.3 nm, 20° C.]: 0.0813 CCZU-2-F 4.00%γ₁ [mPa · s, 20° C.]: 86 CCZU-3-F 13.00% d · Δn [20° C., μm]: 0.50AUUQU-2-F 8.00% Twist [° C.]: 90 CDU-2-F 9.00% V₁₀ [V]: 1.39 CDU-3-F6.00% CC-3-V1 12.00% CC-3-V 18.00% CCH-35 4.00% CGUQU-2-F 8.00%AGUQU-2-F 5.00%

EXAMPLE M8

CC-5-V 8.00% S → N [° C.]: <−40.0 CC-3-V1 11.00% Clearing point [° C.]:+85.5 CCH-35 5.00% Δn [589.3 nm, 20° C.]: 0.0952 ACQU-1-F 10.00% γ₁ [mPa· s, 20° C.]: 126 ACQU-2-F 9.00% d · Δn [20° C., μm]: 0.50 ACQU-3-F2.00% Twist [° C.]: 90 CCP-30CF₃.F 5.00% V₁₀ [V]: 1.21 CCP-20CF₃ 2.00%CCP-30CF₃ 8.00% CCP-40CF₃ 6.00% CGU-3-F 5.00% PGU-2-F 6.00% CCP-V-13.00% CGUQU-2-OT 10.00% CGUQU-3-OT 10.00%

1. Liquid-crystalline medium based on a mixture of polar compounds ofpositive dielectric anisotropy, comprising one or more compounds of theformula I

and one or more compounds of the formula IA

where the proportion of the compounds of the formula I in the medium isat least 18% by weight, and in which the individual radicals have thefollowing meanings: R¹ is an alkenyl radical having from 2 to 8 carbonatoms, R² is H, an alkyl radical having from 1 to 15 carbon atoms whichis halogenated, substituted by CN or CF₃ or unsubstituted, where, inaddition, one or more CH₂ groups in these radicals may each,independently of one another, be replaced by —C≡C—, —CO—, —CH═CH—, —O—,

or

in such a way that O atoms are not linked directly to one another, X¹ isan alkyl radical, alkenyl radical, alkoxy radical or alkenyloxy radical,each having up to 6 carbon atoms, in the case where a=1 also F, Cl, CN,SF₅, SCN, NCS or OCN, X² is F, Cl, CN, SF₅, SCN, NCS, OCN, a halogenatedalkyl radical, halogenated alkenyl radical, OCF₃, OCHF₂, or halogenatedalkenyloxy radical, each having up to 6 carbon atoms, Z¹ and Z² areeach, independently of one another, —CF₂O—, —OCF₂— or a single bond,where Z¹≠Z², a is 0 or 1, and

are each, independently of one another,

L¹⁻⁴ are each, independently of one another, H or F, with the provisothat formula IA is not

in which n is 1-15.
 2. Liquid-crystalline medium according to claim 1,comprising one, two or more compounds of the formulae IA-1,IA-2, IA-4 toIA-17, and IA-19 to IA-30

in which R² is as defined in claim
 1. 3. Liquid-crystalline mediumaccording to claim 1, comprising one or more compounds of the formulaeI-1 to I-5

in which alkenyl is an alkenyl radical having from 2 to 8 carbon atomsand alkyl is a straight-chain alkyl radical having 1-15 carbon atoms. 4.Liquid-crystalline medium according to claim 1, additionally comprisingone or more compounds of the formulae II, III, IV, V and VI

in which the individual radicals have the following meanings: R⁰ is H,n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, eachhaving up to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, alkenyl,alkenyloxy or alkoxy having up to 6 carbon atoms, Z⁰ is —C₂F₄—, —CF═CF—,—CH═CF—, —CF═CH—, —C₂H₄—, —CH═CH—, —O(CH₂)₃—, —(CH₂)₃O—, —(CH₂)₄—,—CF₂O—, —OCF₂—, —OCH₂— or —CH₂O—, Y¹⁻⁴ are each, independently of oneanother, H or F, r is 0 or 1, and the compound of the formula II is notidentical with the compound of the formula I.
 5. Liquid-crystallinemedium according to claim 4, wherein the proportion of compounds of theformulae IA and I to VI together in the mixture as a whole is at least50% by weight.
 6. Liquid-crystalline medium according to claim 1,additionally comprising one or more compounds of the formulae Ea to Ef

in which R⁰ is H, n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy oralkenyl, each having up to 9 carbon atoms.
 7. Liquid-crystalline mediumaccording to claim 1, additional comprising one or more compounds of theformulae IIa to IIg

in which R⁰ is H, n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy oralkenyl, each having up to 9 carbon atoms and the compound of theformula IIa-IIg is not identical with the compound of the formula I. 8.Liquid-crystalline medium according to claim 1, it additionallycomprising one or more compounds of the formulae RI to RVII

in which R* is n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyloxy,each having up to 9 carbon atoms, and alkyl and alkyl* are each,independently of one another, a straight-chain or branched alkyl radicalhaving 1-9 carbon atoms.
 9. Liquid-crystalline medium according to claim1, wherein the proportion of compounds of the formula IA in the mixtureas a whole is from 5 to 40% by weight.
 10. Electro-opticalliquid-crystal display containing a liquid-crystalline medium accordingto claim
 1. 11. A liquid crystal medium according to claim 1, whereinsaid medium has a nematic phase down to −40° C., a clearing point above75° C., and a dielectric anisotropy values Δ∈ of ≧6.
 12. A liquidcrystal medium according to claim 1, wherein said medium has a flowviscosity ν₂₀ at 20° C. of <19 mm²·s⁻¹, a rotational viscosity γ₁ at 20°C. of <120 mPa·s, and a nematic phase range of at least 110° .
 13. Aliquid crystal medium according to claim 2, wherein said medium containsone or more compounds selected from formulae IA-2, IA-5,IA-6,IA-14, andIA-15.
 14. A liquid crystal medium according to claim 2, wherein saidmedium contains one or more compounds selected from formula IA-15. 15.Liquid-crystalline medium according to claim 1, wherein said mediumcontains one or more compounds selected from formulae IA-31 to IA-54:


16. Liquid-crystalline medium according to claim 2, wherein R² informulae IA and IA-1, IA-2, IA-4 to IA-17, and IA-19 to IA-30 is H,straight-chain alkyl having from 1 to 7 carbon atoms, 1E-alkenyl or3-alkenyl.
 17. Liquid-crystalline medium according to claim 15, whereinR² in formulae IA and IA-31 to IA-54 is H, straight-chain alkyl havingfrom 1 to 7 carbon atoms, 1E-alkenyl or 3-alkenyl. 18.Liquid-crystalline medium according to claim 1, wherein said mediumadditionally comprises one or more compounds selected from formulae VIIto XIII

in which R⁰ is H, n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy oralkenyl, each having up to 9 carbon atoms, X⁰ is F, Cl, halogenatedalkyl, halogenated alkenyl, halogenated alkenyloxy, or halogenatedalkoxy, each having up to 6 carbon atoms, Y¹ and Y² are each,independently of one another, H or F, and Y³ and Y⁴ are each,independently of one another, H or F.
 19. Liquid-crystalline mediumaccording to claim 6, wherein the proportion of the compounds of theformulae Ea to Ef is 10-30% by weight.